Modelling Energy Recovery from Wastewater Treatment by Anaerobic Digestion (Published)
In this study, attempt was made to model gas production process from an anaerobic digestion of sewage sludge in a treatment plant. Apart from the issue of environmental cleanup this process of sewage treatment offers, it has become a viable tool to solving energy problems that exist in many parts of the world. Nigeria has much wastewater and this constitutes environmental pollution when channelled to the freshwaters body. Some wastewater; domestic and industrial, has to be treated before channelling them into waterways and in doing this, biogas can be tapped from the system if anaerobic digesters are designed and incorporated into the treatment plants. In this study, this process of biogas production was modelled to ascertain the amount of energy that can be recovered from wastewater treatment plant, for economic usage in the operation of the treatment plant and municipal consumption. To achieve this objective, equation was derived and its application yielded a positive result. Results from two different experimental reactors, reactors 1 and 2 (see Table 4.2 above) were used in comparison with the model reactors to investigate performance of the model. Figure 4.1 shows the gas yield for the different reactors investigated. Statistical analysis of the overall results shows that model reactor 1 has a coefficient of correlation (CORR) of 0.95, this demonstrate a good fit with the experimental results obtained from reactor 1. However, a mean absolute percentage error (MAPE) and root mean square error (RMSE) of 2.15 and 7.49 respectively, was recorded during this process. These values indicate a significantly low error of estimates and shows that the model is reliable.Similarly, model reactor 2 gave a CORR of 0.96 with errors of estimate (MAPE) of 1.34 and RMSE OF 3.12. Meanwhile, it can be observed that both experimental reactor 1 and 2 have a slightly higher values of gas yield than their corresponding model reactors. This trend is rather good in relation to safety in gas production estimate using the model. An overestimating model would be misleading and give a false data when such is needed for energy generation design and operation. . The biogas yield obtained were used to power a micro gas turbine in order to determine electrical energy output from the system, a process that have now been commercialized for economic benefits. Equation 3.24 was derived and its consequent solution, equation 3.27 was used for that purpose. Figure 4.6 shows the energy output for experimental reactor 1. The result obtained shows a close fit between the turbine output and the model output. Precisely, a CORR value of 0.96 was obtained with a small error of estimate of 2.34 and 8.00 respectively for MAPE and RMSE. Similarly, figure 4.7 shows energy output for experimental reactor 2. In this, the coefficient of correlation was found to be 0.94 with MAPE and RMSE being 2.15 and 3.55 respectively. Figure 4.8 and 4.9 shows the energy output for model reactors 1 and 2 respectively. The CORR, MAPE and RMSE were 0.95, 3.78 and 5.51 respectively for model reactor 1 while a similar value of 0.97, 1.73 and 5.02 were recorded for model reactor 2 respectively. In all, a very good correlation values was obtained to show that energy generation from treatment plant can be modelled given the biogas yield data. It should be noted that turbine plant operational mechanism may vary slightly depending on their capacities; consequently, an updated recalibration of the model would be necessary.
Keywords: Anaerobic reactor, Digestion, Energy, Sewage, Treatment.